1. Field of the Invention
The present invention relates to a method and apparatus for forming finished and semi-finished blanks for lenses. More particularly, the invention relates to a method and apparatus for forming blanks for opthalmic lenses from a polymerizable monomeric liquid which has been activated by a suitable catalyst.
2. Discussion of the Prior Art
Within the past twenty-five years, significant strides have been made in techniques for the manufacture of various types of lenses, including spectacle lenses, from plastic materials. When compared to the traditional glass lenses, plastic lenses offer the following significant advantages:
1. The weight is only 40 to 50 percent that of glass making possible the manufacturing of larger spectacle lenses than is feasible using glass.
2. Light transmission is 5 to 8 percent better than that of glass.
3. The tendency to fogging is reduced 60 to 75 percent because of the low thermal conductivity.
4. The impact resistance of many plastics is greater than that of case-hardened glass.
5. Fragmentation, when it does occur, usually yields larger and more obtuse segments than occurs with broken glass thereby making the plastic lenses much safer for use in eyeglasses.
6. Plastics are less affected than glass by high-velocity particles and welder's splatter, making plastic lenses highly desirable for use in safety glasses.
7. Fewer rings are apparent in strong minus lenses.
8. Production by molding or casting eliminates surfacing procedures.
Of all the materials suggested for use in the manufacture of plastic lenses, allyl diglycol carbonate shows perhaps the greatest promise. This material is a colorless organic monomeric liquid of low viscosity and low volatility. When a catalyst such as benzoyl peroxide or isopropyl percarbonate is dissolved in the monomer and heat is applied, the liquid gradually thickens to form a soft gel. With further heating, this gel hardens into an insoluable, infusible, clear, colorless solid.
Because of the unique properties of the thermoset plastics manufactured from allyl diglycol carbonate, the material is ideally suited for the production of opthalmic lenses. The material, when properly processed, is very strong, has excellent optical and dimensional stability and is highly solvent and temperature resistant. Certain characteristics inherent in allyl diglycol carbonate monomers and the catalysts or initiators used therewith, however, makes the design of large scale production processes using these materials most difficult. For example, the reaction between the monomer and the catalysts is highly exothermic, making precise process temperature control most difficult. Moreover, in large scale manufacturing operations, temperature control problems are magnified since the exotherm is nonuniform during the curing process with a very large exotherm occurring near the end of the polymerization cycle. Additionally, isopropyl percarbonate is a highly unstable chemical at room temperature and therefore must be kept at a temperature not above 0.degree. F.
When allyl diglycol carbonate and isopropyl percarbonate are used in the manufacture of opthalmic lenses, it is particularly important to closely control processing temperatures. Failure to do so results in material warpage, excessive shrinkage, and unacceptable material linearity and molecular weight. Additionally, unless the processing temperatures are maintained within closely defined limits, the material produced will exhibit poor optical clarity, poor color stability and unsatisfactory abrasion resistance.
Because of control problems inherent in processing allyl diglycol carbonate and its catalysts, prior art lens manufacture has, for the most part, been accomplished by batch processing techniques with no continuous processing method having to date been successfully developed. Typically, prior art batch manufacturing methods involve mixing the materials, placing the liquid mixture in suitable molds and then curing the material in large ovens having vertically spaced apart mold-carrying racks. Due to the exothermic character of the materials, it is virtually impossible to achieve uniformity of temperature within the ovens. Consequently, in certain locations within the oven the material cures too rapidly and in other locations too slowly. The end result is gross nonuniformity of the end product. Although attempts have been made to circulate air through the ovens to control temperature, such attempts have been generally unsuccessful. This is largely due to the nonuniform addition of heat of exotherm to the system by the material itself as it cures.
In addition to the aforementioned drawbacks of prior art methods, batch processing techniques are inherently cumbersome, inefficient, and expensive. For example, in order to avoid temperature shock to the material, the batch processing ovens must be shut down and allowed to cool after each curing cycle. This constitutes inefficient use of the equipment and markedly increases the cost of manufacture.
The method and apparatus of the present invention for the continuous manufacture of opthalmic lenses effectively overcomes the drawbacks of prior art techniques in a highly novel manner. In accordance with the method of the present invention, the material curing process is accomplished by passing the molds containing the activated monomer through an elongated curing apparatus made up of a plurality of individually controlled air-circulating units. As the molds are automatically carried through the apparatus of the invention they are sequentially exposed to collimated streams of air maintained at a predetermined elevated temperature. The streams of air serve to controllably heat the material within the molds and at the same time carry away the heat of exotherm. In this way, the temperature of the material within each mold can be uniquely and precisely controlled throughout the entire curing or polymerization cycle. The system is designed to operate continuously and automatically thereby maximizing the use of the equipment and minimizing labor costs.